12 



would have seen some facts that would help explain it. Similar 

 clouds perhaps began to form over your head in the early after- 

 noon and drifted away toward the east, developing into thunder 

 storms many miles to the east of you. 



On such a day as this, the air near the ground is so damp that 

 it gives up vapor easily, as you can prove by allowing a glass of 

 ice water to stand on a table and watching the drops of water 

 gather there, causing the glass to " sweat" (Fig. i). The sun 

 beats down upon the heated ground and the surface becomes like 

 a furnace, so that the air near the ground is warmed. 



Air that is warm is lighter than cool air, and, being lighter, 

 will rise, for the heavy cool air will settle and push it up, as a 

 chip of wood will rise in a pail of water, because it is lighter 

 than the water which pushes it to the top This is why the 

 warm air rises from a furnace, or a stove, or a lamp. It is the 

 reason why the hot air rises through a house chimney ; undoubt- 

 edly you can find other illustrations, as ventilation, and can find 

 abundant opportunity to prove that warm air will rise. 



The warm, moist air near the ground becomes so light that the 

 heavy air above settles down and pushes it up, so that an upris- 

 ing current of air is formed above the heated ground, much as 

 an uprising current of hot air rises through the chimne}' when the 

 stove is lighted. Rising thousands of feet into the skj^ it reaches 

 such a height, and finally comes to a place so cool, that some of 

 the vapor must be condensed, forming fog particles, and then 

 makes a cloud. 



On such a day, if you will watch a cloud, you will notice 

 that its base is fiat (Fig. 7) ; and this marks the height above 

 ground where the temperature of the atmosphere is low enough 

 to change the vapor to fog particles. Of course the air still rises 

 somewhat above this base and continues to get cooler and to 

 have more and more vapor condensed. This makes a pile of 

 clouds resting on a level base, but with rounded tops (Fig. 7). 

 Sometimes the base of these summer clouds, called cumulus 

 clouds, is a mile above the ground and their tops fully a mile 

 higher than this. 



Just as on the mountain side the drops grow larger until they 

 must fall, so here, fog particles grow to drops of such a size 



